This invention relates to motor and fan assemblies especially adapted for use in electric appliances, and is particularly directed to fans and fan assemblies wherein the fan is formed of a plastic material.
The assemblage of a fan on the shaft of an electric motor for use in an electric appliance is, of course, quite well known. The present invention is particularly directed to the use of such fans and fan assemblies for electric vacuum cleaners, although it will be apparent that the invention may also be advantageously employed in other appliances.
When a fan is employed in a vacuum cleaner, it is conventionally mounted to the shaft of the motor. This mounting may be effected, for example, by means of a threaded pulley on the end of the shaft, the face of the pulley exerting mounting pressure on the face of the hub of the fan. Conventional belts couple the pulley to the working components of the vacuum cleaner.
Under normal operating conditions, heat generated, for example, in the motor, is readily dissipated so that no damage may result to the fan. On occasion, however, the heat generated may become extensive. This may occur, for example, if the motor is stilled, or if the belt engaging the pulley slips excessively. If the fan is of a conventional metallic material, for example, a die-cast fan, the generation of such additional heat will generally result in no damage to the fan.
For a number of reasons it may be desirable to employ a plastic fan in vacuum cleaners. For example, plastic fans may be readily molded as unitary members, thereby reducing their cost. In addition, some of the newer plastic materials, such as polycarbonate, are particularly adaptable to this use since, when properly molded, such materials are substantially "unbreakable."
It has been found, however, that plastic fans are more subject to damage as a result of heat. Thus, the plastic materials are in general thermal insulators, in the sense that their coefficient K of heat transfer is substantially less than that of previously employed metallic materials. This can result in the buildup of the temperature at the hubs of the fans, particularly under adverse conditions. The additional heat may result in the "cold flow" of the plastic material since the hub of the fan is subjected to mechanical mounting forces. As a consequence, the material of the hub of the fan may flow over the mounting nut or pulley so that effective mounting of the fan on the shaft no longer exists. The fan may thereby become subject to slippage and damage to result in a further buildup of heat in the assembly with consequent damage to the fan and also the possibility of damage to the motor itself.
The present invention is directed to the provision of means for overcoming the above problem when the fan of an electric appliance is formed of a plastic material. Specifically, in accordance with the invention, the mounting hole in the hub of the fan has a larger diameter than that of the shaft and insulating washers are mounted in annular recesses at each end of the mounting hole. The washers have internal diameters substantially the same as that of the shaft so that the fan may be centered and rigidly mounted to the shaft. The washers, although of a heat insulating material, may be expediently of a material that is not subject to cold flow. The provision of the washers and the spacing between the fan and the shaft of the motor prevents the passage of sufficient heat to the fan to effect the cold flow of the fan. As a consequence, damage to the fan, and resultant damage to the remainder of the assembly, may be avoided under adverse conditions even though the fan is of a plastic material.